Production facilities for making asphalt concrete to be used as a paving composition are well-known. Input feed materials for these facilities will include aggregate materials and asphalt cement. The aggregate materials may be provided in the form of virgin aggregate materials, and/or recycled asphalt product (“RAP”) comprised of crushed asphalt concrete, and/or recycled asphalt shingles (“RAS”) comprised of comminuted asphalt roofing shingles. If RAP and/or RAS is included in the input feed materials, these components will also provide an additional source of asphalt cement.
Conventional asphalt concrete production plants typically employ a rotating dryer drum having a burner at one end. Into this drum, virgin aggregate materials and/or RAP and/or RAS are introduced for heating. One end of the drum is elevated above the other, so that the input feed materials are moved along the drum from the upper end through the heated gases generated by the burner in either parallel flow (i.e., the hot gases and the material being heated move in the same direction) or counter-current flow (i.e., the hot gases and the material being heated move in opposite directions) to an outlet at the lower end. A separate mixer, such as a rotating drum mixer or a pugmill, is employed to mix the heated and dried aggregate materials with liquid asphalt cement. Another type of asphalt concrete production plant employs a dryer/mixer that dries and heats the aggregate material and also mixes it with asphalt cement. One such type of dryer/mixer is the DOUBLE BARREL® brand dryer/mixer that is sold by Astec, Inc. of Chattanooga, Tenn. This dryer/mixer includes a generally cylindrical fixed outer drum and a heating chamber comprised of a generally cylindrical inner drum that is adapted to rotate with respect to the outer drum. A burner at one end of the inner drum heats aggregate material by direct exposure to the hot gases generated, and the heated aggregate material is discharged from the inner drum into the outer drum where it is mixed with asphalt cement.
Because some conventional mixers expose liquid asphalt cement and/or RAP and/or RAS aggregate materials to the high-temperature gases used for drying and heating the aggregate materials and to the steam generated in the drying process, emissions of smoke and volatile organic components (“VOC”) are stripped from the light oil fractions of the asphalt cement components. In order to prevent these emissions from being discharged to the atmosphere, it has been deemed desirable, when only virgin aggregate materials are used, to either direct the emissions into the burner for incineration, or filter the emissions from the plant exhaust gases and condense them for disposal. Even though counter-current flow is more thermally efficient than parallel flow, conventional asphalt concrete production plants that process only RAP and/or RAS are generally operated in a parallel heat flow arrangement, where the aggregate materials to be heated and dried are carried through the dryer in the same direction as the heating gases, in order to minimize smoke and VOC emissions. In addition, exposure of high proportions of RAP and/or RAS aggregate materials to the high-temperature gases used for drying and heating the aggregate materials and to the steam generated in the drying process causes oxidation of the liquid asphalt on the RAP and/or RAS, which results in degrading the asphalt and any pavement materials made with it. This reduces the number of applications for which high-RAP content or high-RAS content asphalt concrete is considered suitable.
It would be desirable if a method and apparatus for producing asphalt concrete could be provided that would limit the emission of undesirable smoke and VOC. It would also be desirable if such a method and apparatus could be provided that would be more thermally efficient than conventional systems, especially when used to process aggregate materials including RAP and/or RAS. It would also be desirable if such a method and apparatus could be provided that would minimize the oxidation of asphalt cement in high-RAP content and/or high-RAS content asphalt concrete, thereby making such products suitable for more paving applications.